用环形激波聚焦实现爆轰波直接起爆的数值模拟

利用基元反应模型和有限体积法对环形激波在可燃气体中聚焦实现爆轰波直接起爆进行了数值模拟。研究结果表明，标准状态下的氢气-空气混合气体在马赫数为3.1以上的环形激波聚焦产生的高温高压区作用下会诱发可燃气体的直接起爆形成爆轰波，爆轰波与激波和接触间断相互作用产生了复杂的波系结构；爆轰波爆点位置在对称轴上并不是固定的点，而是随着初始激波马赫数的变化而发生移动；可燃气体初始温度和压力对起爆临界马赫数都有影响，但是初始温度的影响大得多。     

可燃气体中激波聚焦的点火特性

数值模拟了二维平面激波从抛物面上反射在可燃气体中聚焦的过程，研究了形 成爆轰波的点火特性. 对理想化学当量比氢气/空气混合气体，在初始压强20kPa的条件下， 马赫数2.6-2.8的激波聚焦能产生两个点火区：第1个点火区是反射激波会聚引起的，第 2个点火区是由入射激波在抛物面上发生马赫反射引起的. 这种条件下流场中会出现爆燃转 爆轰，起爆点分别分布在管道壁面、抛物反射面和第2点火区附近. 起爆机理分别为激波管 道壁面反射、点火诱导激波的抛物面反射和点火诱导的激波与第2点火区产生的爆燃波的相 互作用. 不同的点火和起爆过程导致了不同的流场波系结构，同时影响了爆轰波传播的波动 力学过程.     

可燃气体中激波与障碍物作用在下游形成爆轰波的数值研究

对平面激波和单个矩形障碍物作用的过程进行了数值模拟,研究了反射产生的上行爆轰波在下游可燃气体中形成爆轰波的过程。数值结果表明,下游爆轰波形成过程主要有2种模式:爆轰波直接绕射和绕射波在上壁面反射,这和已有的实验结果是一致的。通过研究下游爆轰波的形成过程受入射激波马赫数、混合气体的压力和管道尺度的影响,分析了上游爆轰波向下游传播的波动力学过程,讨论了2种形成过程的作用规律和控制因素,阐明了下游爆轰波的形成规律。     

高速弹丸诱导斜爆轰激波结构实验研究

斜爆轰推进系统在高超声速推进领域具有广阔的应用前景,其释热迅速、比冲高、燃烧室结构简单的优点吸引研究人员的持续关注.然而,斜爆轰的地面试验同时涉及到高速试验环境模拟、燃料与氧化剂混合、高温燃烧流场结构测量等技术难点,当前国内外系统的试验研究仍然十分有限,难以支撑斜爆轰发动机的研制.为了研究自持传播的斜爆轰激波结构与波面流动特性,基于爆轰驱动二级轻气炮开展了高速弹丸诱导斜爆轰实验研究,使用直径30 mm球头圆柱形弹丸发射进入充满氢/氧可燃混合气体的实验舱中以起爆斜爆轰波,并采用两种阴影技术对实验流动结构进行测量.实验中在不同速度、不同充气压力下观察到三种弹丸诱导激波结构,即激波诱导燃烧、弹丸起爆爆轰波和相对弹丸驻定的斜爆轰波,实验舱充气压力下降则会造成爆轰横波尺度增加与波面流动失稳.实验中,斜爆轰激波角与理论分析结果吻合较好,弹丸气动不稳定带来较大的弹丸攻角会对激波角测量带来一定偏差.通过对斜爆轰波波面法向传播速度的测量发现,随着远离弹丸,斜爆轰传播速度由弹丸飞行速度衰减至接近实验气体CJ速度,弹丸速度的降低会加速斜爆轰波传播速度的衰减.     

两种不同气体中的高速爆燃波及其向爆轰的转变

对2C2H2 5O2及2C2H2 5O2 80%Ar两种可燃混合气体中的高速爆燃波及其向爆轰的转变过程进行实验研究.高速爆燃波由孔栅干涉爆轰波的方法直接生成,观测手段则以高速转鼓摄影获取孔栅近场流场x-t纹影图,以传感器追踪波面的后继发展.研究发现,两种气体中的爆燃波具有迥异的特性.前者燃烧波面在较低初压条件下为层流结构,而较高初压下为湍流结构,向爆轰转变点可以延伸至下游较长距离;后者在不同初压条件下燃烧波面无明显差异,爆轰的再次形成只能在孔栅下游近场内建立.两种气体中高速爆燃波的维持和爆轰转变过程均非纯粹激波压缩所致,湍流输运在其中起着必不可少的作用.分析显示,激波压缩效应对纯氧炔气体的高速爆燃和DDT贡献较小,湍流输运占主导地位;而氩气稀释气体较为稳定,缺乏自行衍生剧烈湍流燃烧的能力,因而激波压缩和外界扰动对其高速爆燃传播和爆轰转变起十分重要的作用.     

激波与爆轰物理研究进展专辑 序

激波作为气体动力学最具特色的基本物理现象之一,表现出强间断与非线性的物理特征.激波能够在超/高超速气流内部诱导漩涡,生产复杂的气体物理过程;激波能够诱导热化学反应,构成了高温气体动力学和凝聚态爆轰动力学的学科基础;激波还能够压缩可燃气体和可燃气固两相物质实现其自点火,形成能够以超声速自持传播的燃烧激波—–爆轰波.激波与爆轰物理的相关研究已经有一百多年的历史了,在各种爆炸现象的预防与弱化及其应用与防护方面;在天体物理领域的超新星和恒星的演化乃至宇宙大爆炸理论探索方面;在现代武器技术和医疗技术领域的应用方面发挥了日益重要的作用.特别是     

激波与爆轰波对撞的数值模拟研究

用二阶精度NND差分格式和改进的二阶段化学反应模型模拟了爆轰波与激波的对撞过程,研究了不同强度入射激波对爆轰过渡区域的影响. 当对撞激波较弱时,透射爆轰波演变主要受流动膨胀作用的影响,可划分为对撞影响区、爆轰恢复区和稳定发展区3个阶段. 在爆轰恢复区和稳定发展区,前导激波压力经历一个过冲、然后向稳定爆轰过渡的过程,表现了爆轰波熄爆和再起爆的物理特征. 当对撞激波较强时,可燃混合气体的高热力学参数导致了更高的化学反应活化程度,形成了弱爆轰向稳定爆轰的直接转变.      

序

激波作为气体动力学最具特色的基本物理现象之一,表现出强间断与非线性的物理特征.激波能够在超/高超速气流内部诱导漩涡,生产复杂的气体物理过程;激波能够诱导热化学反应,构成了高温气体动力学和凝聚态爆轰动力学的学科基础;激波还能够压缩可燃气体和可燃气固两相物质实现其自点火,形成能够以超声速自持传播的燃烧激波|{爆轰波.激波与爆轰物理的相关研究已经有一百多年的历史了,在各种爆炸现象的预防与弱化及其应用与防护方面;在天体物理领域的超新星和恒星的演化乃至宇宙大爆炸理论探索方面;在现代武器技术和医疗技术领域的应用方面发挥了日益重要的作用.特别是近十几年来,随着高超声速和空天飞行器研制的进展,激波与爆轰研究在近空间飞行器的气动布局,先进高超声速推进技术,高焓气体流动的气动力/热问题研究方面得到了广泛的重视和深入的探索.      

超音速来流中爆轰波衍射和二次起爆过程研究

预爆管技术被广泛地应用在爆轰波发动机的起爆过程中，但是在超音速来流中基于预爆管技术起始爆轰波的研究并未被广泛地开展。基于此，本文中数值研究了横向超音速来流对半自由空间内爆轰波的衍射和自发二次起爆、及管道内的衍射和壁面反射二次起爆两种现象的影响。数值模拟的控制方程为二维欧拉方程，空间上使用五阶WENO格式进行数值离散，采用带有诱导步的两步链分支化学反应模型。所模拟的爆轰波具有规则的胞格结构，对应于用惰性气体高度稀释过的可爆混合物中形成的爆轰波。结果表明:在半自由空间内，在本文所模拟的几何尺寸下，爆轰波并未成功发生二次起爆现象，但是爆轰波的自持传播距离随着横向超音速来流强度的增强而增加。在核心的三角形流动区域外，波面诱导产生了更多的横波结构；在管道内，横向的超音速来流在逆流侧对出口气流产生了压缩作用，能有效提高波面压力，因此反射后的激波压力也比较高。在同样的几何尺寸下，爆轰波在静止和超音速(Ma=2.0)气流中分别出现了二次起爆失败和成功两种现象，这是由于在超音速来流中化学反应面的褶皱诱导产生了横波结构，横波与管壁以及其他横波之间的碰撞提高了前导激波的强度，并最终促进了爆轰波在超声速流主管道内的成功起始。     

用于爆轰驱动的射流起爆实验研究

爆轰驱动激波风洞的自由来流模拟范围与驱动气体的爆轰极限密切相关，爆轰极限越宽则模拟范围越大。驱动气体一般是通过点火管进行起爆的，提高点火管的起爆能力可以拓宽爆轰极限。为了提高点火管起爆能力，就点火管口径、点火气体爆轰敏感性和单/双点火管3种因素的影响进行了实验研究。在不同的点火管初始条件下，对驱动段波速进行了测量。结论如下:（1）提高点火管口径可以显著提升起爆能力；（2）点火气体爆轰敏感性对起爆能力有影响，点火管为缩径内型面时，低敏感性气体起爆能力更强，点火管为等径内型面时则低敏感性气体和高敏感性气体的起爆能力大体持平；（3）在保证射流同步的前提下，双点火管能够提高起爆能力，为保证射流同步性需使用化学恰当比的氢氧混气等爆轰敏感性强的点火气体。     

激波绕射碰撞加速诱导爆轰的数值模拟

基于单步化学反应的Euler方程和对激波(爆轰波)、接触间断具有良好捕捉效果的Roe/HLL混合格式以及自适应网格技术,模拟了激波在方形管中与方块障碍物相互作用,并发生绕射碰撞来诱导爆轰的过程.结果表明,弱激波在绕经方块时,形成上、下绕射激波并在方块尾部发生碰撞,生成局部高温高压点,可加快爆轰的形成;而当管内阻塞比超过...     

DDT and detonation waves in dust-air mixtures

This paper summarizes the studies of DDT and stable detonation waves in dust-air mixtures at the Stosswellenlabor of RWTH Aachen. The DDT process and propagation mechanism for stable heterogeneous dust detonations in air are essentially the same as in the oxygen environment studied previously. The dust DDT process in tubes is composed of a reaction compression stage followed by a reaction shock stage as the pre-detonation process. The transverse waves that couple the shock wave and the chemical energy release are responsible for the propagation of a stable dust-air detonation. However, the transverse wave spacing of dust-air mixtures is much larger. Therefore, DDT and propagation of a stable detonation in most industrial and agricultural, combustible dust-air mixtures require a tube that has a large diameter between 0.1 m and 1 m and a sufficient length-diameter ratio beyond 100, when an appropriately strong initiation energy is used. Two dust detonation tubes, 0.14 m and 0.3 m in diameter, were used for observation of the above-mentioned results in cornstarch, anthraquinone and aluminum dust suspended in air. Smoked-foil technique was also used to measure the cellular structure of dust detonations in the 0.3 m detonation tube. Received 11 February 2000 / Accepted 1 August 2000     

Evolution of detonation propagation through an annular gap into a combustion chamber

Some results of theoretical studies of detonation processes in combustible gaseous mixtures are discussed for a model geometry of large combustion chambers of detonation engines in the case of mixtures of hydrogen and oxygen-enriched air. The effect of geometric characteristics on the operation of pulse detonation engines is analyzed. In particular, the propagation of detonation waves in tubes of small diameter to larger volumes and the evolution of detonation under the action of converging shock waves are considered.     

Transition of plane overdriven detonation wave to the Chapman-Jouguet regime

The asymptotic laws of behavior for plane, cylindrical, and spherical infinitely thin detonation waves were found in [1, 2] for increasing distance from an igniting source in those cases in which the waves changed into Chapman-Jouguet waves as they decayed. It was shown that the plane overdriven detonation wave approaches the Chapman-Jouguet regime asymptotically, while the transition of the cylindrical or spherical strong detonation wave into the Chapman-Jouguet wave may occur at a finite distance from the initiation source.Similar conclusions are valid for the propagation of stationary steadystate detonation waves which arise with flow of combustible gas mixtures past bodies.However, numerous experiments [3, 4] on firing bodies in a detonating gas show that the overdriven detonation wave which forms ahead of the body decays and decomposes into an ordinary compression shock and a slow combustion front. To establish why the wave does not make the transition to the Chapman-Jouguet regime, in the following we consider the propagation of a plane detonation wave and account for finite chemical reaction rates. We use the very simple two-front model (ordinary shock wave and following flame front). Conditions are found for which transition to the Chapman-Jouguet regime does not occur. We first consider the propagation of an unsteady plane wave and then the steady plane wave. It is found that for all the mixtures used in these experiments transition to the Chapman-Jouguet regime is not possible within the framework of the assumed model.     

Mechanism of deflagration-to-detonation transitions above repeated obstacles

Experiments are carried out to investigate the mechanism of the deflagration-to-detonation transition (DDT). Because, this mechanism has relevance to safety issues in industries, where combustible premixed gases are in general use. A stoichiometric gas of oxygen and hydrogen (oxy-hydrogen) is ignited in a tube, repeated obstacles are installed, and the DDT behaviours are visualized using a high-speed video camera. The pitch and height of the repeated obstacles and the initial pressure of the oxy-hydrogen premixed gas are varied in an attempt to obtain the optimum conditions that cause DDT a short distance from the ignition source. The experiments identified DDT as being essentially caused by one of the following mechanisms: (1) A deflagration wave is accelerated in terms of a vortex, which is generated behind the obstacle, and the flame acceleration induces a secondary shock wave. Eventually, the shock–flame interaction ahead of the obstacle causes DDT via a very strong local explosion. (2) Each shock wave generated by relatively weak local explosions between the obstacles is not sufficient to cause DDT directly, but DDT results from an accumulation of shock waves. The detonation induction distance is also examined, taking into account the physical and chemical parameters of the obstacles and the oxy-hydrogen premixed gas.     

Visualization of deflagration-to-detonation transitions in a channel with repeated obstacles using a hydrogen–oxygen mixture

The deflagration-to-detonation transition in a 100 mm square cross-section channel was investigated for a highly reactive stoichiometric hydrogen oxygen mixture at 70 kPa. Obstacles of 5 mm width and 5, 10, and 15 mm heights were equally spaced 60 mm apart at the bottom of the channel. The phenomenon was investigated primarily by time-resolved schlieren visualization from two orthogonal directions using a high-speed video camera. The detonation transition occurred over a remarkably short distance within only three or four repeated obstacles. The global flame speed just before the detonation transition was well below the sound speed of the combustion products and did not reach the sound speed of the initial unreacted gas for tests with an obstacle height of 5 and 10 mm. These results indicate that a detonation transition does not always require global flame acceleration beyond the speed of sound for highly reactive combustible mixtures. A possible mechanism for this detonation initiation was the mixing of the unreacted and reacted gas in the vicinity of the flame front convoluted by the vortex present behind each obstacle, and the formation of a hot spot by the shock wave. The final onset of the detonation originated from the unreacted gas pocket, which was surrounded by the obstacle downstream face and the channel wall.     

Detonation formation in H $_2$-O $_2$/He/Ar mixtures at elevated initial pressures

In this paper the formation of detonation in H-O/He/Ar mixtures at elevated initial pressures was investigated in an initiation tube for a detonation driver with an exploding wire as the ignition source. In most experiments the detonation wave was formed by a DDT process in which a reactive shock wave accelerates behind the leading shock wave and eventually leads to the onset of detonation. The onset position was found to be at the leading shock wave or behind it. Only in very sensitive mixtures at high initial pressure the direct initiation of detonation was observed. The influence of ignition energy, initial pressure and composition on the detonation induction distance was determined. The results show that the detonation induction distance increases with the decrease of ignition energy and initial pressure and with the increase of the mole fraction of helium or argon. With the same mole fraction, argon increases the induction distance more than helium. In the facility utilized the DDT upper and lower limits of hydrogen in H-O mixtures are in the ranges from 36 to 40 % and from 78 to 82 %, respectively, and the upper limits for helium and argon in stoichiometric H-O mixtures are 40 % and 36 %, respectively. High pressure peaks generated by the DDT process were measured, especially in mixtures near the DDT limits. Statistical results show that such peak pressures can be up to 6 times of the CJ-pressures. Received 1 March 2000 / Accepted 25 May 2000     

Detonation diffraction in combustible high-speed flows

Detonation propagating in a T-shaped tube with quiescent and moving hydrogen/oxygen/argon mixtures is numerically examined based on the Euler equations with detailed finite-rate chemistry using the fifth-order weighted essentially non-oscillatory scheme. When diffracted in a quiescent combustible mixture, the detonation wave propagating from the bottom of the T-shaped tube is influenced by the corner rarefaction waves and decays into a non-reacting shock. Subsequently, the decoupled shock reflects irregularly from the top wall. Through several reflections back and forth between the top and bottom walls, a planar detonation is finally re-established. When the combustible mixture in the horizontal part flows from the left to the right, the detonation products ejected from the vertical tube will retard the flow, generating a compression flow upstream and a rarefaction flow downstream. The disturbed detonation on the left side is stronger than that on the right side. The final planar detonation in the upstream direction propagates faster than the Chapman–Jouguet (CJ) detonation with compressed, fine cellular structures, whereas the detonation in the downstream direction propagates more slowly than the CJ detonation with elongated, coarse cellular structures. The details of the transient behavior of diffracting detonation in high-speed flows are discussed.